Previously published data from this laboratory demonstrates that the heavy metal, cadmium, mimics the effects of estradiol in the estrogen responsive breast cancer cells, MCF-7. These studies also suggested that the effects of cadmium were mediated by the estrogen receptor (ER). Data presented in this grant suggest that cadmium activates the ER by forming a coordination complex with 4 amino acids in the hormone binding domain. Binding studies demonstrate that the metal binds with high affinity to the ER (kd=10-10 M). Preliminary data also suggest that arsenite activates the estrogen receptor. In ovariectomized animals, low doses of cadmium increased uterine wet weight and promoted mammary gland development. In ovariectomized rats treated with the carcinogen dimethylbenzanthracene (DMBA), cadmium replaced estradiol as a promoter of mammary tumorigenesis. Taken together, these results suggest that environmental exposure to cadmium activates the estrogen receptor and leads to an increased risk of breast cancer. The aims of this grant are two fold: 1, to define the role of cadmium in the development of breast cancer; and 2, to determine whether arsenite is estrogenic. Aim 1 will test whether cadmium mimics the effects of estradiol in ovariectomized animals and/or enhances the effects of estradiol in intact animals. Effects of cadmium on the expression of estrogen responsive genes in target organs and on circulating amounts of hormone will be determined. Aim 1 will also test whether cadmium mimics and/or enhances the effects of estradiol in the DMBA- and N-methylnitrosourea (NMU)-induced rat mammary tumor models. The studies in aim 2 are designed to determine whether arsenite activates the estrogen receptor. In these studies the effects of arsenite on the expression of estrogen responsive genes will be measured. The ability of an antiestrogen to block the effects of arsenite will be tested. Aim 2 will test the hypothesis that arsenite activates the ER by a direct interaction with cysteines, histidines, and basic amino acids in the hormone binding domain.